1. Field of the Invention
This invention relates to an electronic identification tag interrogation system method in which an electronic tag is attached to a person or object for identification purposes. The present method resolves the problem of collisions between replies from various electronic identification tags received by a sensing portal.
2. Description of the Prior Art
Electronic identification tag systems are known for tracking personnel and property. The electronic identification tags track personnel and property as they enter and exit a monitored area through various portals. Such electronic identification tags are also used to detect unauthorized access by personnel through portals to secured areas.
The tracking of personnel and property is accomplished by monitoring the location of an electronic identification tag attached to the personnel or property. Interrogation means provided at the portals to a confined area monitor the ingress and egress of personnel and property through each portal. By monitoring the ingress and egress of personnel and property through various portals, the location of the personnel or property in a much larger facility can be determined.
In order to track the personnel or property, a transmitter provided at a portal sends an interrogation message which is received by the electronic identification tags affixed to personnel or property within the search field of the portal. Usually, the search field of a portal is directed to a confined area controlled by the portal. A transmitter provided in the electronic identification tag responds to the interrogation message by sending a reply message. Frequently, the interrogation message from a portal includes a portal identifier which instructs the electronic identification tag to reply with a similar portal identifier included in the reply message. The portal identifier instructs only the proper portal to process the reply, thereby eliminating processing of a reply message by an unintended portal which is within the transmission range of the electronic identification tag. Thus, if two portals receive a reply signal from an electronic identification tag, the reply message includes means for determining which portal should process the reply message.
Where the number of personnel or packages being tracked is large, the possibility exists that a number of reply messages from the electronic identification tags affixed to the personnel or packages will collide when received by the portal. To minimize problems caused by colliding reply signals, prior art interrogation systems provide a time variant delay in the electronic identification tag responses. This time variant delay is at least pseudo-random to provide diversification of responses and minimize collisions of the reply messages.
The prior art time variant delay diversification scheme is proposed for the situation in which two tags are in the field of an interrogating portal and reply at exactly the same time. This is usually not a problem for access portals where a tag is used to unlock a gate or activate an escapement to allow entrance to an area. The tag in this case is activated by a portal field that is limited in range of activation. Otherwise, a tag in back of the first can open the lock by a process known as "tailgating". Physical means are needed to limit the "tailgating" to guarantee that consecutive tags are not mistaken for each other. This is accomplished by shaping the interrogating field of influence at a portal. In such an instance, a time variant delay diversification scheme to prevent collisions of reply messages may not be necessary.
A different type of portal reading device is necessary for loading parcels or bags, such as of mail, onto trucks. In this case, a portal reading device located at the top of the truck opening records the loading or unloading of parcels. The portal reading device consists of two interrogation heads which indicate that the parcel has been moved from an area just outside the truck opening onto the truck bed. The use of two interrogation heads provides the ability to determine that the parcel or bag was actually loaded onto the truck bed. Because of the proximity of the two interrogation heads, there is a possibility that the fields of the separate heads may overlap. To overcome this problem, the overlap area may be fashioned to become a region of ineffective communication and, therefore, a dead zone. A microprocessor receives input from the interrogation heads and monitors whether the parcels have been loaded onto the truck beds. During interrogation, the replies from the electronic identification tags may collide. Diversification of the replies is necessary to prevent the continuation of the collisions.
As stated previously, the prior art employs a time delay system to achieve diversification of replies. A problem arises with such a diversification scheme when a large volume of parcels may possibly be expected to be present within the field of the interrogation heads. In order to accommodate a large volume of parcels, an equal number of diverse time delays must be provided. Consequently, every time an interrogation signal is transmitted, the monitoring system must wait for every possible time delayed response to arrive before retransmitting an interrogation signal. If the interrogation system is provided with 100 time delays, the transmitter must wait for all 100 diverse time-delayed responses to be sent before retransmitting an interrogation signal. The system must wait for reception of all possible time-delayed response signals regardless of the actual volume of parcels present in the interrogation field. In other words, when time delay diversification is used, the interrogation system must wait for the possibility of every conceivable reply regardless of whether or not a parcel is present to provide such a reply. Consequently, there is a need for an interrogation system which eliminates the frequently unnecessary wait for the last conceivable time delayed reply.